This invention relates to an inverter type X-ray apparatus, and more particularly to a circuit for decreasing inverter current in such an apparatus.
In a conventional X-ray apparatus connected to a commercial AC power source, it is a common practice that a regulated voltage obtained by selectively changing the position of slidable brushes disposed on the secondary sides of a voltage regulating transformer or by selectively changing over output taps disposed on the secondary side of a voltage regulating transformer is raised by a high voltage transformer, and such a high voltage is applied to an X-ray tube after being rectified.
On the other hand, with the recent remarkable progress of power semiconductor elements, an inverter type X-ray apparatus using such semiconductor elements for the purpose of power control has been developed and proposed recently. The inverter type X-ray apparatus developed recently is advantageous over the conventional X-ray apparatus in that its power control response is very quick as compared with that of the conventional X-ray apparatus using a voltage regulating transformer as described above, because of the use of semiconductor elements for attaining the power control. Therefore, the inverter type X-ray apparatus is advantageous in that a tube voltage can also be easily regulated during X-ray exposure, so that the tube voltage can be accurately set at any desired level suitable for X-ray exposure.
A prior art, inverter type X-ray apparatus has a construction as, for example, disclosed in Japanese Unexamined Patent Publication No. 54-118787 (1979). The construction of part of the prior art X-ray apparatus cited above will be described with reference to FIG. 8.
Referring to FIG. 8, a full-wave rectifier circuit 1A connected to a chopper circuit 4A to provide a predetermined DC voltage. This chopper circuit 4A is composed of a chopping transistor 4b, a smoothing reactor 4a, a free-wheel diode 4c and a smoothing capacitor 4d. The connection is such that, in the off-period of the chopping transistor 4b, current from the smoothing reactor 4a flows through a loop which is traced from the smoothing reactor 4a.fwdarw. smoothing capacitor 4d.fwdarw. freewheel diode 4c to the smoothing reactor 4a. An inverter 5A inverts the DC output voltage of the smoothing capacitor 4d into a corresponding AC voltage. This inverter 5 is composed of transistors 5a and 5b. In the inverter 5A, the transistors 5a and 5b are alternately turned on to apply an AC voltage to a high voltage transformer 6A. The voltage raised by the high voltage transformer 6 is applied to an X-ray tube 8 after being rectified in a full-wave rectifier circuit 7.
As shown in FIG. 8, the output voltage of the full-wave rectifier circuit 1A is applied to the chopper circuit 4A normally after being rectified by the smoothing circuit composed of a reactor 2 and a condenser 3, however, such smoothing circuit is omitted in the embodiment of Japanese Patent Unexamined Publication No. 54-118787.
In the prior art, inverter type X-ray apparatus having a construction as described above, the chopper circuit 4A is used for regulating the tube voltage. There is the following relation between an input voltage V.sub.R and an output voltage V.sub.C of the chopper circuit 4A: ##EQU1## where fc is equal to 1/T.sub.C is the operating frequency of the chopper circuit 4, Tc is the period of the frequency, and Ton is the on-duration of the transistor 4b. Therefore, a predetermined output voltage can be obtained as desired by changing the value of Ton. Hereinafter, the ratio Ton/Tc will be called a duty ratio.
However, since there is the relation Ton&lt;Tc, the output voltage V.sub.C is necessarily lower than the input voltage V.sub.R in the illustrated arrangement, as apparent from the expression (1). Therefore, in order to provide a predetermined tube voltage, the winding ratio (referred to hereinafter as a step-up ratio) of the high voltage transformer 6A must be selected to be sufficiently large. On the other hand, in order to supply a predetermined output current from the high voltage transformer 6A, an input current, which is as large as a value obtained by multiplying the tube current by the winding ratio of the high voltage transformer 6A, must be supplied to the primary winding of the high voltage transformer 6A. Thus, the larger the winding ratio of the high voltage transformer 6A, the larger is the input current that must be supplied to the high voltage transformer 6A for providing the predetermined output current, that is, the current flowing through the transistors 5a to 5d of the inverter 5A.
Suppose, for example, that the X-ray apparatus is connected to a commercial AC power source of single-phase 200 [V]. The value of the smoothed output voltage of the full-wave rectifier circuit 1A is generally an average of the values of the AC input voltage applied under a loaded condition. Therefore, the terminal voltage V.sub.R of the smoothing capacitor 3 under the loaded condition is given by ##EQU2## Suppose that the maximum value of the duty ratio (=Ton/Tc) of the chopper circuit 4A is 0.9. Then, the output voltage V.sub.C of the chopper circuit 4 is expressed as follows: EQU V.sub.C =0.9.times.V.sub.R =162 [V] (3)
In order to apply a tube voltage of 150 [kV] to the X-ray tube 8 when the output voltage V.sub.C of the chopper circuit 4 is 162 [V], the winding ratio K of the high voltage transformer 6A is given by the following expression: ##EQU3##
The output of X-ray apparatus of this type has been greatly increased up to now. In order to supply a tube current of 1000 [mA] to the X-ray tube 8, the value of an input current I.sub.T1 that must be supplied to the high voltage transformer 6A is calculated as follows: EQU I.sub.T1 =1000 [mA].times.K=926 [A] (5)
Therefore, the transistors 5a and 5b incorporated in the inverter 5A are required to be capable of controlling a large current as large as about 1,000 [A]. Semiconductor elements capable of controlling such a large current are quite expensive. In addition, the resistance Rl of the wiring connected to the inverter 5 and to the inputs of high voltage transformer 6A increases a power loss Wl which is expressed as follows: EQU Wl=Rl.times.I.sub.T1 2 (6)
Thus, the prior art, inverter type X-ray apparatus has had the problem that an increase in the input current I.sub.T1 supplied to the high voltage transformer 6A results in a corresponding increase in the power loss Wl due to the wiring resistance Rl on the input sides of the inverter 5A and high voltage transformer 6A and also in a corresponding reduction of the operating efficiency of the X-ray apparatus.